A common misconception in writing is that you are writing a spec so you can sell it. This is not the case. A sale, while nice, is unlikely in the current climate. My advice is to write a sample that communicates your ability to work in a given genre.
My advice for beginning writers can be summed up thusly:

Your story should be between 95-115 pages.

Your story should have a likable star part with a clear and recognizable arc.

Your story should hit the familiar beats while paradoxically feeling fresh and original.

Your story should be a strong example of a single, commercial genre.

Nothing in your script should be longer than 4 lines. You can break this rule 5 times.

It should explore cynicism, but reaffirm optimism (unless it’s a horror movie, in which case kill everybody).

Don’t world build too much – if your universe begins to resemble Warcraft or Star Wars, you might want to write a novel.

You must give a shit about what you write. If you can’t give a shit within these rules, then mainstream screenwriting might not be for you.

It’s not that this is the best way, or the only way, or even the right way. There is no right way, but there’s nothing egregious in this advice (again, so long as you give a shit).

IS THIS A BY THE NUMBERS FORMULA?
No. A common misconception with Save the Cat and other Mad Lib-type approaches to movies is that they’ve done all the thinking for you, and you just need to fill in the blanks. If it were that easy, they would have software to write screenplays. The great challenge of screenwriting is to take a familiar form like this and find a way to make it personal. Find what you’re trying to say, then use this story to say it (this topic merits 10,000 words on it’s own, but for now just remember that there are no easy answers). You have to give a shit (for more on this, see below).

IF EVERYONE DOES THIS WON’T THIS APPROACH LOSE VALUE?
Absolutely. No approach works forever, but for now, given that most people can’t internalize simple advice like this, if you do try it this way, it’ll help you stand out. Screenwriting advice is like rock, paper, scissors. There’s no best approach, the battlefield is always changing.

IS THIS GUARANTEED TO WORK?
Nothing is guaranteed to work. Being a professional screenwriter is like making it to the NBA. There’s a hoop on every playground, but only 350 NBA players in the world. This is just some simple advice that represents what I think gives you the best shot of showcasing your ability to write to the average reader.

HOW DO I DO THIS?
There’s no best way. You will find your method. I have my ways, which are listed on my website.

REALLY, WHY ARE WE DOING IT THIS WAY?
There’s a kind of sameness to most movies. This isn’t due to a great conspiracy, it’s because humans are particular in terms of the kind of culture they will accept. A good story tends to have some sense of unity, causality, and obedience to some kind of theme. The most successful writers are the ones who find joy in what they do. There’s a misconception that Michael Bay or Brett Ratner are some kind of sellouts. Not so, they love the kinds of movies they make as much as Tarantino or Scorcese do theirs. If this advice makes you gag, don’t follow it. If you see the sense of it, try writing within these parameters. It’s an approach that’s as good as any, better than most.